Introduction to Thermodynamics

Transcription

1 Introduction to Thermodynamics

2 Thermodynamics Rub your hands together for 15 seconds. Are your hands warm? Thermal energy

3 Thermodynamics The study of the effects of work, heat flow, and energy on a system Movement of thermal energy Engineers use thermodynamics in systems ranging from nuclear power plants to electrical components. SURROUNDINGS SYSTEM BOUNDARY

4 Thermal Energy versus Temperature Thermal Energy is kinetic energy in transit from one object to another due to temperature difference. (Joules) Temperature is the average kinetic energy of particles in an object not the total amount of kinetic energy particles. (Degrees) Temperature #1 Temperature #2 Heat

5 Temperature Scales Scale Freezing point of water Boiling point of water Celsius 0 C 100 C Fahrenheit 32 F 212 F Kelvin 273K 373K Matter is made up of molecules in motion (kinetic energy) An increase in temperature increases motion A decrease in temperature decreases motion Absolute Zero occurs when all kinetic energy is removed from a object 0 K = -273 C

6 Thermodynamic Equilibrium Thermal equilibrium is obtained when touching objects within a system reach the same temperature. When thermal equilibrium is reached, the system loses its ability to do work. Zeroth Law of Thermodynamics: If two systems are separately found to be in thermal equilibrium with a third system, the first two systems are in thermal equilibrium with each other. Object #1 (Thermometer) Object #1 Object #2 Object #2 Object #3

7 Thermal Energy (heat) Transfer The transfer or movement of thermal energy Most common types of transfer Convection Conduction Radiation 100% efficiency is unattainable ALL processes are irreversible

8 1st Law of Thermodynamics Law of energy conservation applied to a thermal system Thermal energy can change form and location, but it cannot be created or destroyed. Thermal energy can be increased within a system by adding thermal energy (heat) or by performing work in a system.

9 1st Law of Thermodynamics Example: Using a bicycle pump Pumping the handle results in what? Applying mechanical energy into the system Mechanical energy is converted into thermal energy through friction (the pump becomes hot) The total increase in internal energy of the system is equal to what? The applied mechanical energy istockphoto.com

10 2nd Law of Thermodynamics Thermal energy flows from hot to cold istockphoto.com istockphoto.com When you touch a frozen pizza with your hand, thermal energy flows in what direction? Hand Pizza When you touch a cooked pizza with your hand, thermal energy flows in what direction? Pizza Hand

11 2nd Law of Thermodynamics Entropy is the measure of how evenly distributed heat is within a system. - A system tends to go from order to disorder Order Disorder Firewood has low entropy (molecules in order) when stacked and high entropy when burning (molecules in disorder). The total amount of energy in the world does not change, but the availability of that energy constantly decreases.

12 Thermal Energy Transfer Convection The transfer of thermal energy by movement of fluid (liquid or gas) When fluid is heated, it expands, becomes less dense, and rises. Boiler heating systems circulate heat throughout a home without pumps through the use of convection.

13 Thermal Energy Transfer Conduction The transfer of thermal energy within an object or between objects from molecule to molecule A metal spoon placed in a hot cup of soup will feel warm to your hand. The heat from the soup is conducted through the spoon. istockphoto.com

14 Thermal Energy Transfer Equations Q = m c ΔT Q = energy transfer (Joules) m =massof thematerial(kilograms) c =specific heat capacity of the material (J/kg C) ΔT = change in temperature

15 Thermal Energy Transfer Equations Q k A ΔT P= P= Δt L P L k= A ΔT P = rate of energy transfer (Watts) Q = energy transfer (Joules) Δt =change in time(seconds) k = thermal conductivity A = area of thermal conductivity L = thickness of material ΔT = difference in temperature

16 Calculating Energy Transfer Calculate the energy transferred when a block of aluminum at 80.0 C is placed in 1.00 liter (1kg) of water at 25.0 C if the final temperature becomes 30.0 C. Step 1. List all known values Q Mass of water = m water = 1 kg Specific heat capacity of water = c w = J 4184 o kg C change in temperature = J c Al = 900. o kg C change in temperature = ΔT water = 30.0 C 25.0 C = 5.0 C ΔT Al = 80.0 C 30.0 C = 50.0 C

17 Calculating Energy Transfer Step 2. List all unknown values Q = energy transferred m Al = mass of the Al block Step 3. Select equations to solve unknown values Q =m c ΔT Q Al =Qwater Step 4. Solve for Q water J kg C o Q water = (1.00kg) C= 21,000 J gained o

18 Calculating Energy Transfer Step 5. Solve for m Al Q (lost) = Q Al water (gained) = 20,920 J Q = m c ΔT Al Al Al Q 20,920J Al m Al = = c Al ΔT J 900. o o 50.0 C kg C m = 0.46 kg =460g Al

19 Calculating Energy Transfer Calculate the energy transfer in a wall section measuring 2m by 1m by 0.04m thick J with a thermal conductivity of s m C Opposing sides of the wall section have a temperature of 10 C and 5 C after one hour. Q Step 1. List all known values Area of thermal conductivity = A = 2m * 1m = 2m 2 Thermal conductivity = k =0.10 Thickness of material = L = 0.04m J o s m C Difference in temperature = ΔT = 10 C - 5 C = 5 C Change in time = Δt = 1 hour = 3600s

20 Calculating Energy Transfer Step 2. List all unknown values P = Rate of energy transfer Q = Energy transfer Step 3. Select equations to solve unknown values Q ka T P = P = Δt L Step 4. Solve in terms of Q Q = P Δt Step 5. Combine equations ka ΔT Q = ( ) Δt L

21 Calculating Energy Transfer Step 6. Apply known values ka ΔT Q= Δt L Q = J s m C m 5 C 3600s 0.04m Q = 90,000J 2

22 U-Value Coefficient of Heat Conductivity The measure of a material s ability to conduct heat P U= AΔT Metric system W m 2 C U.S. customary system Btu ft 2 hr F

23 R-Value Thermal Resistance of a Material The measure of a material s ability to resist heat The higher the R-value, the higher the resistance 1 P R= U= U AΔT Bulk R-value = R-value Object 1 + R-value Object 2 + = Total R-Value

24 Calculating R-Value Determine the R-value of the wall cavity below Fiberglass batt (R=19) 5/8 in. drywall (R=0.56) Wall cavity R-value = in. foil-faced polyisocyanurate (R=7.20) 1 in. air space (R=0.17) Brick 2 ¼ x 3 ½ x 8 (R=0.8) What is the R-value at a stud location? =15.6 2x6 construction (2x6 R=6.88)

25 Thermal Energy Transfer Radiation The process by which energy is transmitted through a medium, including empty space, as electromagnetic waves Stefan s Law All objects lose and gain thermal energy by electromagnetic radiation. P =σae(t 4 -T 4 ) net 2 1 istockphoto.com Electromagnetic waves transfer to food and other matter P = radiated energy transfer W -8 σ = Stefan's constant = x10 m 2 K 4 e emissivity constant T temperature in Kelvin A = area

26 Thermal Energy Transfer Prior to dressing for school, a student watches the morning weather to decide what clothes to wear. The bedroom is 65ºF and the student s skin is 91.4ºF. Determine the net energy transfer from the student s body during the 15.0 minutes spent watching the morning weather. Note: Skin emissivity is 0.90, and the surface area of the student is 1.30m 2. Step 1. List all known values Area = A= 1. 30m Emissivity constant Stefan s constant Bedroom temperature Skin temperature Change in time 2 =e=0.90 = σ = =T =65 F =T 2=91.4 F W m K 2 4 istockphoto.com = t = 15.0 minutes = 900.s

27 Thermal Energy Transfer Step 2. List all unknown values P = Rate of energy transfer Q = Energy transfer istockphoto.com Step 3. Select equations to solve unknown values 4 4 P net =σae(t 2 -T 1 ) Q = P Δt 4 4 Step 4. Apply known values to P net =σae(t2 - T 1 ) 4 4 Solve for(t 2- T 1) 91.4 F = K 65 F = K

28 Thermal Energy Transfer Step 4 (continued). Apply known values to P 4 4 net =σae T2 -T1 Step 5. Combine equations and solve Q = PΔt Q =103.92W 900.s Q = 93,500J istockphoto.com 93,500J of energy are transferred from the student s body during the 15 minutes spent watching the morning weather.

29 Applications of Thermal Energy

30 Examples of Solar Energy All images were obtained from the following URL:

31 Geothermal Energy Energy generated from the thermal energy stored beneath the Earth s surface Also refers to the heat that is collected from the atmosphere; for instance, near the oceans

32 Resources Energy Information Association. (n.d.). Energy kid s page. Retrieved March 23, 2008, from McGraw-Hill dictionary of engineering. (2nd ed.). New York, NY: McGraw-Hill. NASA. (2008). Glenn research center. Retrieved March 23, 2008 from National Renewable Energy Laboratory. (2007). TroughNet. Retrieved March 23, 2008, from U.S. Department of Energy. (2008). Solar energy technologies program. Retrieved March 23, 2008, from